Conventionally semiconductor chip packages use molded underfill and lead frame or ceramic substrate, but recently many micro chips adopt wafer-level package. Wafer-level package uses a chip cap to protect the sensitive circuitries or vulnerable structures within chips such as the suspended movable devices of MEMS (Micro Electro Mechanical System) sensing chips from impacts of external environment. Many MEMS sensing chips such as accelerometers or pressure sensors usually are wafer bonded to a glass or silicon wafer with a plurality of recesses in order to protect their sensing structures or diaphragm and also provide structures such as hermetic seal and through-silicon via (TSV).
Commonly used wafer bonding techniques comprise fusion bonding, anodic bonding and bonding with intermediates such as eutectic bonding or polymer bonding. Since fusion bonding and anodic bonding are limited to silicon to silicon or silicon to silicon dioxide bonding and silicon to sodium-containing glass bonding respectively and these two bonding techniques require low roughness of wafer surfaces, these two bonding techniques could not be applied widely. Therefore, wafer bonding techniques using compatible interlayers or bonding pairs are adopted more frequently. In such a case, glass frit is widely used for consumer electronics. However, glass frit requires using screen printing to form bond ring, so it is impossible to form bond rings with width less than 100-200 μm. This limitation poses great challenges to the trend of continuous miniaturizing of chip sizes. In a case of using polymers (such as BCB or photoresist), photolithography processes can be used to form bond rings with great precision, so width of bond ring can be reduced significantly. However, polymers may out gas when exposed to high temperature and their bonding strength is weaker, so they may affect the reliability of products.
In an eutectic bonding process by bringing specific metals into contact under a relatively low temperature to form eutectic phase, metallic layers would be formed on a MEMS wafer and a cap wafer and patterned. After applying a load to bring two wafers into contact, heating them to a temperature higher than eutectic temperature and keeping the temperature for a while, these two wafers would be bonded together. In this kind of eutectic bonding process, metals commonly used in or compatible with semiconductor manufacturing processes are often chosen. For example, U.S. Pat. No. 7,943,411 taught using an Al—Ge eutectic bonding process to bond a cap wafer on a MEMS wafer. Since the eutectic temperature for Al—Ge is 419° C., process temperature would be increased to 430-450° C. in order to form a stable bonding. However, such a high temperature would adversely affect some film stacks and the thermal stress therefrom would lead to deformation or functional failure of the sensing film/films. U.S. Pat. No. 5,668,033 disclosed using an Au—Si eutectic bonding process to bond a cap on an accelerometer chip. Since the eutectic temperature for Au—Si is 363° C., process temperature would be slightly reduced to 390-410° C. However, this process comes with some disadvantages such as higher cost of Au and challenges of native oxide formed on the Si surface. Therefore, there is a need to develop an eutectic bonding technique compatible with semiconductor manufacturing processes, using lower eutectic temperature and with lower cost to perform capping process on a MEMS apparatus. U.S. Pat. No. 6,229,190 disclosed using an Ag—Sn eutectic bonding process. In such a process, Ag or Sn are formed on a pressure sensor and a cap wafer respectively and then a capping and bonding process is conducted on the pressure sensor. Since the eutectic temperature for Ag—Sn is 221° C. that is much lower than the eutectic temperature for Al—Ge and Au—Si, it could significantly avoid the thermal stress issues mentioned before. Aside from this benefit, its rather low cost (much lower than Au) makes it a promising technique. However, this bonding technique suffers from low melting point (about 230° C.) and low mechanical strength of the brittle Sn. If the package product made by Ag—Sn eutectic bonding process still contains high ratio of pure Sn, this pure Sn not only would reduce the strength of bonding interface but also would damage the package product while ensuing process temperature (such as process in reflow furnace involving a temperature of 250° C.) is higher than its melting point 230° C.